The present invention generally relates to solid propellants, and, more particularly, to plasticizers used therein for purposes of improved processing, lower temperature flexibility and improved mechanical properties.
Plasticizers are used in rocket propellants for two general purposes which are processing assistance by incorporation of fluid materials in the propellant mix, and also the improvement of the low temperature flexibility and mechanical properties. There are some objections to the use of plasticizers in propellants which are associated with the tendency of plasticizer molecules to migrate or evaporate. Either of these processes results in chemical changes in composition which are bad for the propellant and for other inert parts of the rocket motor. On long term storage, for example, changes in mechanical properties of the propellant occur, particularly near the linear-rocket motor propellant bond. Secondary negative effects associated with the use of some plasticizers include crystallization at low temperatures, a tendency to soften the propellant excessively at high temperatures, and the migration of the plasticizer into the liner from uncured propellant much more rapidly than it does from a cured system. Consequently, it is desirable to have a plasticizer which does not migrate rapidly even from uncured propellant.
For a time, it was thought these problems could be circumvented by using larger plasticizer molecules which presumably would not migrate. One of the largest molecules used as a plasticizer in the rocket propellant industry is a material known as ZL-496. ZL-496 is a polybutadiene with a molecular weight of approximately 3,000. This material is used because the molecular size is quite large and it is believed that polymer chain entanglement would prevent migration. Experimentation with insulation taken from solid propellant motors, approximately five years old, indicated otherwise. One of the samples was taken from insulation in contact with the propellant, and the other was taken from the insulation that was separated from the propellant. After appropriate sample preparation technique, transmission FT-IR spectra was collected. These experiments definitely showed that even plasticizer molecules as large as ZL-496 do have some migration tendency which will ultimately act to shorten the shelf life of rocket motors.
The above observations show a need for internal plasticizers which will not migrate on aging or cause propellant to soften excessively at high temperatures.